In recent years a so-called Mid-Foil SWAS vessel was developed, as disclosed in U.S. Pat. No. 5,794,558, which uses a submerged underwater displacement hull or lifting body to provide lift to the craft in conjunction with any other parts of the vessel which generate lift. The lifting body differs from a hydrofoil in that the enclosed volume of the lifting body provides significant displacement or buoyant lift as well as hydrodynamic lift, whereas the lift of a hydrofoil is dominated by only hydrodynamic lift. In the course of continuing development work, the particular shape of such lifting bodies was studied in detail in order to improve their performance and adapt and integrate their use to a wide range of marine craft.
More specifically, as disclosed in U.S. Pat. No. 6,263,819, it was found that the submerged bodies of marine vessels, when operated at shallow submergence depths, such as is the case for SWAS and Mid-Foil vessels, can be adversely effected by the displacement of the free water surface caused by the body's volume and dynamic flow effects. The interaction of that displacement of the free surface relative to the body's shape had not been adequately accounted for in the prior art structures. It is believed that this inadequacy of existing prior art submerged bodies for marine vessels is the result of the fact that submerged and semi-submerged marine vessels have historically been designed to operate at great depths relative to their underwater body thickness, as with submarines or hydrofoils.
A typical submarine is essentially a body of revolution-shaped hull which has three dimensional waterflow about it, but which is designed to operate normally several hull diameters or more below the free water surface. Thus, the displacement of the free surface of the water by operation of the hull at such depths is minimal and does not effect the operation of the vessel. On the other hand, hydrofoils are simply submerged wings with predominately two-dimensional flow and are designed typically to produce dynamic lift as opposed to buoyant or hydrostatic lift.
The displacement of water at the free surface by a submerged body is detrimental to a marine vessel's hydrodynamic performance with the impact varying as a function of the body's shape, submergence depth, speed and trim. For example, the free surface effects can significantly reduce lift in the body or even cause negative lift (also referred to as sinkage) to occur. Resistance to movement through the water by free surface effects is generally greater than if the submerged hull were operating at great depths; and pitch movements caused by the displacement of the free water surface vary with speed and create craft instability.
According to the teachings of U.S. Pat. No. 6,263,819 (hereinafter the “'819 patent”), these problems are overcome by a low drag underwater submerged displacement hull defined from two parabolic shapes. The periphery of the hull when viewed in plan is symmetrical and defined by a first parabolic form (or parabolic equation) with the form defining the leading edge of the hull. The longitudinal cross-section of the hull is formed of foil shaped cross-sections which are defined as cambered parabolic foils having a low drag foil shape and providing a generally parabolic nose for the hull. Generally, each longitudinal cross-section of the hull parallel to the longitudinal or fore and aft axis of the hull has a symmetrical cambered parabolic foil shape with the cross-section along the longitudinal axis of the hull having the maximum thickness and the cross-section furthest from the centerline of the hull having the minimum thickness. In plan, the hull has a stem or trailing edge which is defined by either a straight line, a parabolic line, or a straight line fared near its ends to the side edges of the plan parabola shape.
It is an object of the present invention to provide a submerged lifting body which can be employed on the bow of various marine vessels to maximize performance of the vessel by creating a high lift to drag ratio (L/D), i.e., low drag, at operational speed.
Another object of the present invention is to provide a submerged lifting body for use on the bow of various marine vessels which improves performance of the vessel at operational speed while creating a dynamically stable vessel.
Yet another object of the present invention is to provide submerged lifting bodies for use on the bow of various marine vessels which can increase the efficiency of these vessels by reducing hydrodynamic drag.
A still further object of the invention is its increase in the efficiency of a ship's hull through use of a bow mounting lifting body.